The invention relates to a process for the reduction of oxygen in aqueous chlorine- and/or chloride-containing solutions in the presence of a catalyst comprising nitrogen-doped carbon nanotubes.
Considerable quantities of aqueous hydrogen chloride solutions are obtained in the chemical industry. This applies particularly in the production of aromatic and aliphatic isocyanates. The recycling of chlorine from the solutions containing hydrogen chloride is usually carried out industrially by means of a hydrogen chloride electrolysis. These hydrogen chloride electrolyses involve the reduction of oxygen on the cathode side.
To reduce the overvoltages or cell potentials needed for the reduction of oxygen, use is frequently made of catalysts for the reduction of oxygen, This is intended, in particular, to reduce the energy costs of the process. In many cases, catalysts comprise noble metals or compounds comprising noble metals, e.g. platinum and compounds comprising platinum, so that the processes are generally very costly.
U.S. Pat. No. 6,149,782 discloses a catalyst comprising rhodium sulphide (RhSx) as essentially catalytically active component by means of which oxygen can be reduced in aqueous, chlorine-containing solutions. Furthermore, U.S. Pat. No. 6,149,782 discloses that the use of rhodium sulphide is advantageous because the change from pure noble metals (in particular platinum) results, inter alia, in dissolution of the catalytically active component of the catalyst occurring only to a greatly reduced extent. It is explained that some contact of the chlorine- and/or chloride-containing solution with the catalytically active component of the catalyst cannot be prevented and the contact is therefore the cause of this leaching as a result of complexes and/or salts of the metals of the catalytically active component being formed and leached from the catalyst. A disadvantage of the catalyst according to U.S. Pat. No. 6,149,782 is that rhodium is a rare and thus expensive material, so that essentially the same economic disadvantages as in the case of platinum or other noble metals and their compounds stand in the way of use of the catalysts disclosed. In addition, leaching of the catalytically active component rhodium sulphide can likewise not be prevented entirely since rhodium and its compounds also form stable salts and/or complexes with chlorine and these are leached to a small extent from the catalyst, so that the latter becomes depleted in catalytically active material and is deactivated as a result. In a process based on this catalyst, the deactivated catalyst would therefore have to be replaced at regular intervals, so that the abovementioned disadvantages of rhodium make the process economically unattractive.
U.S. Pat. No. 7,074,306 discloses that catalysts based on rhodium sulphide have a further technical disadvantage in that at high current densities they catalyze the formation of hydrogen, which leads to safety problems in the operation of processes utilizing such catalysts. It is therefore disclosed that this disadvantage can be alleviated by the use of platinum together with rhodium sulphide. In this way, according to the disclosure, the accumulation of hydrogen in the O2 gas stream is prevented even at high current densities.
However, a process using such catalysts/catalytically active components is just as economically disadvantageous as that mentioned above, since the combination of rhodium and platinum combines the economic disadvantages of the two.
It is also stated that the ability of the catalyst to reduce the cell voltage or the overvoltage for the reduction of oxygen is decreased solely by changing the solution from a solution containing sulphuric acid to a solution containing hydrochloric acid. An improvement in the maximum possible periods of operation of the process without the necessity of changing the catalyst material as a result of the addition of platinum is not disclosed.
US 2006/0249380 discloses further suitable catalytically active components which can be employed in catalysts for the reduction of oxygen in aqueous chlorine- and/or chloride-containing solution. Apart from the abovementioned noble metals, rhodium and platinum, the document also discloses iridium, rhenium, ruthenium and palladium, their sulphides and oxides and also mixed phases, in particular ones containing molybdenum and/or selenium, as possible catalytically active components. A material combination whose catalytic effect is not based on noble or transition metals is not disclosed. It is likewise taught that it is advantageous not to bring any hydrochloric acid into contact with the catalyst since corrosive attack on the catalytically active components disclosed is prevented in this way. Ideally, contact of chlorides and/or chlorine with the catalytically active component should be prevented entirely.
In the light of the disadvantages of the prior art, it is therefore an object of the invention to provide a process for the reduction of oxygen in aqueous chlorine- and/or chloride-containing solutions, which dispenses with the use of expensive noble and/or transition metals as essential catalytically active component, which has no tendency to form hydrogen and does not necessitate replacement of the catalytically active component due to its consumption or deactivation and reduces the cell potential necessary to a value comparable to that in a process in which no aqueous chlorine- and/or chloride-containing solutions are present.